Polyimides in the prior art generally have been produced from highly aromatic monomers. These polyimides have exhibited good thermo-oxidative stability in temperature ranges up to 300.degree. to 400.degree. C. Thus, in applications where elevated temperatures and corrosive conditions exit, polyimides have been extensively employed. However, because polyimides exhibit low percentages of elongation to break, and because they exhibit essentially no recoverable elongation of elasticity, they have not been used for many applications such as seals, sealants, or certain coating applications which required an elastomeric or compliant material.
U.S. Pat. No. 3,812,082 teaches a compliant or highly flexible polyimide produced from a condensation reaction of a dianhydride and a diamine or diisocyanate. These polyimides exhibit an elongation to break of up to 300 percent, and a set at break of 25 percent or less. Thus, where a high performance sealing material is required and it can be cast, these polyimides are very suitable.
A significant advance in a practical route to flexible polyimides is taught in U.S. Pat. No. 3,951,902. In this addition-type poly(Diels-Alder) approach, a melt polymerization reaction yields polymers acceptable for use at temperatures up to 550.degree. F. The polyimides processed by this melt process possess properties equivalent to the solution route employed in U.S. Pat. No. 3,812,082. Thus, a melt process is available to prepare flexible polyimdies which possess a high elongation to break and can be melt extruded or cast into a variety of products including seals, sealants, adhesives and coatings. This availability of melt polymerization and melt product fabrication processes means substantial cost reduction and significant lessening of human hazards and environmental pollution over polyimides produced and processed by conventional solution methods.
U.S. Pat. No. 3,652,511 teaches a water-dispersible polyimide coating formed by reacting maleic anhydride with an aliphatic diamine in solution. The polyimide product can be formed into a hard and solvent resistant film. Elastomeric or high recoverable elongation characteristics are not inherent to the solution-produced polyimide itself and are achieved only by a copolymer reaction such as reacting a sulfhydryl-terminated polymer, such as nitrile rubber, with the maleic unsaturation in the bismaleimide or the acidic side chain. This approach significantly limits or excludes their utility in terms of applications where use in seal, sealant adhesive and coating use is required at temperatures of 250.degree. F. or greater. Also, these resins have pendant carboxyl groups to achieve water solubility which tends to diminish their utility for general engineering applications due to possible unfavorable side reactions in high temperature use environments.
Similarly, U.S. Pat. No. 2,818,405 teaches elastomeric polyimides formed by the equal molar reaction of bismaleimides and free organic diamines. The organic diamines employed in this invention are hydrocarbon or halogenated hydrocarbon segments which restrict temperature performance to 200.degree. F. or below. Also, the technology disclosed in this patent necessitates the use of organic tri- or tetraamines to accomplish cure of the linear imide resin produced initially. Use of the technology described therein severely limits achievement of a broad range of polymer mechanical properties because of the requirement to use 1:1 molecular reactant stoichiometry.